How Would One Go About Explaining The Metric System To One Whom Has Never Used It?

The metric system of measurement is based on powers of ten. To
understand the metric system, it is important to know the meaning of
the prefixes to each base unit.  

• kilo = thousand
• hecto = hundred
• deka = ten
• deci = tenth
• centi = hundredth
• milli = thousandth

Once you have that down...  just do simple math... Provided you know simple math...

It depends on if the person is child or adult, because different concepts might be used to explain it most simple terms, but basically I would begin as:

The metric system is based on a system of 10.   This is just like the decimal system used worldwide.  (Deci means 10) It is MUCH easier to manage and convert than non-metric systems because of this.

Take the meter, for example, a standard measure of length in the metric system.  There are 10 millimeters ( ( 1/thousandth of a meter, milli means 1/1000th), in a centimeter, and there are 100 (10x10) centimeters in a meter (cent prefix means 100) and there are 1000 meters (100 x 10) in a kilometer (kilo means 1000).  (obviously there would be 10 centimeters in a decimeter, and so on and so forth).  

The gram is another example.  It is a measurement of weight.  A milligram is 1/10th of a centigram.  A centigram is 1/100th of a gram, and a gram is 1/1000th of a kilogram.  (And look how smart this was!-- a kilogram in roughly the same weight/volume measure as 100 cubic centimeters of water!  Nice trick, huh?)

Liter is the base measure of volume.  All of the same conversions apply. (another very nicely tied in fact then is that the liter is a measure of capacity equal to the volume occupied by one kilogram of water --see how everything fits so nicely?)

Centigrade, the metric measure of temperature, is another example.  O degrees is freezing.  Every degree above opn this scale is also based on the base 10 system.

See how everything is a multiple of 10 in the system?  That makes conversion very easy.  The metric system is used almost exclusively in science and math, especially in the higher sciences of physics, etc.

I don't think I can explain the whole system here as I would do to someone in person, but there are some web tools that could help.  Here is a site that explains it probably much better than I am doing for you here, and it also has some great links, and they are very understandable.  It also provides links for learners who are more visual and want to see an illustration of what they are learning.  Does it help?

www.internosis.com

Here is a website that is a great converter of all types of things. Hope this helps.


people.virginia.edu

Peace

I would take my time and go step by step to ensure the person really understands it

I'd make analogies to fingers and toes, assuming you or the learner still have all ten, of course!